1. Field of the Invention
The present invention relates to a ladder-type electric filter for use in radio equipment or the like, in which a plurality of piezo resonators are combined in series-parallel to each other.
2. Description of the Related Art
The ladder-type electric filter of the kind described above has a configuration of thick and large-capacity series resonators, thin and small-capacity parallel resonators, and a plurality of terminal plates for electrically, mechanically holding the series and parallel resonators therebetween in a casing, as shown in FIGS. 5 and 6.
Such filters are used in filter circuits of various radio communication equipment, and particularly, when such a filter is used in mobile radio equipment of the phase modulation system type, such as a land mobile radiotelephone, a pocket pager, or the like. This filter is required to be superior in group delay characteristics, in addition to the conventional amplitude characteristics.
Conventionally, in order to respond to the foregoing requirements, the group delay characteristics of the improved ladder-type filter have been promoted by using resonators formed of piezo-ceramics having a small mechanical quality coefficient Qm (hereinafter, simply referred to as a "Qm"). However, there has been a disadvantage in that the insertion loss of the ladder-type electric filter remarkably increases in comparison with a ladder-type electric filter using large Qm resonators.
Small Qm piezo-ceramics resonators x and large Qm piezo-ceramics y have been combined with each other to form a predetermined ladder-type circuit.
FIG. 7 shows the characteristics of various ladder type electric filters in which small Qm resonators x and large Qm resonators y which are combined in series-parallel to each other.
Here, sample 1 was such that small Qm resonators x were disposed in series-parallel to each other at the input side and large Qm resonators v were disposed in series-parallel to each other at the output side, as will be shown later in an embodiment of the invention; sample 2 was such that only the small Qm resonators y were disposed in series-parallel to each other; sample 3 was such that only the large Qm resonators x were disposed in series-parallel to each other; and sample 4 was such that the large Qm resonators v were disposed in series-parallel to each other at the input side and the small Qm resonators y were disposed in series-parallel to each other at the output side.
Amplitude characteristics were judged on the basis of the frequency band width at the position where the frequency was lowered from the center frequency by 3 dB. In view of the fact that the frequency is required to have a predetermined value as to the amplitude characteristics, all the samples may be used with no trouble, although sample 4 is slightly inferior to the others. The insertion loss is large in sample 2. Further, when a large-capacity system is used in a land mobile radiotelephone, the frequency band width of 12.5 kHz is given to every radio channel and, therefore, the attentuation characteristics in a range of .+-.6.25 kHz from the center frequency were examined. As a result, it was found that sample 2 was inferior. Moreover, the group delay characteristics were examined, and it was found that sample 3 was inferior.
When the results were comprehensively judged, it was found that all the values were good in sample 1 and therefore, sample 1 was the most suitable for the filter circuit of a land mobile radiotelephone. Further, it was found that sample 4 could be used.
It was also found that when the ladder-type electric filter in which small Qm resonators x and large Qm resonators y are combined with each other as described above, superior group delay characteristics occur making the filter suitable for a mobile radiotelegraph of the phase modulation system.
When large Qm series or parallel resonators and small Qm series or parallel resonators are combined with each other as described above, there is an advantage in that an increase of the insertion loss is suppressed by the large Qm resonators v and the group delay characteristics are improved by the small Qm resonators x, so that a low insertion loss and good group delay characteristics are realized as a whole. However, the frequency constant of the large Qm resonator y is large, and in order to satisfy the relation of "frequency constant =frequency X length" it is, therefore, necessary to select the external size b of the large Qm resonators y to be larger than the external size a of the small Qm resonators x, as shown in FIG. 6. For example, when the Qm of a resonator in which contour oscillation is generated at the resonance frequency of 455 kHz is 200, the frequency constant of the resonator is 1950, and the external size obtained through the foregoing expression is therefore, 4.5 mm.sup.2. In the case of a resonator having a Qm of 600, on the other hand, the frequency constant is 2150, and the external size is therefore, 4.8 mm.sup.2. Consequently, there arises a new problem in that it is necessary to design the internal size of a casing for housing the resonators having the foregoing external sizes so as to suit the external size of the large Qm resonator. Therefore, the shape of the casing becomes large and it is difficult to use the casing in thin mobile radio equipment. This poses difficulties when reduction in size, weight, and thickness of radio equipment are desired. Pocket telephones, etc., with the above desired properties are in high demand.